Racquets 10 for sports such as tennis, racquetball, and squash typically comprise a handle 12 connected to a head or frame 14 strung with cylindrical nylon or gut strings 16, as shown in FIG. 1. The strings are threaded through openings in the frame 14 and woven to make a grid-like pattern that serves as a ball striking surface. The grid pattern of the weave is created by alternating horizontal and vertical strings, 18 and 20, respectively, in an over and under manner, as shown in FIG. 2, which is a sectional view of FIG. 1 taken along the line A-A'. The woven strings 16 are placed under tension, within the material limits of the frame 14 and strings 16, to provide the rebound and control characteristics of the striking surface desired by the racquet sport athlete. The texture of the weave is related to the shape of the individual strings 16 and the distortion each causes the overlapping string to undergo. The angle of distortion for overlapping circular strings 16 is depicted as .THETA. in FIG. 2.
The string tension, and the rebound and control characteristics influenced thereby, produce a variety of collateral effects during play that are related to string displacement, racquet vibration, and string durability. For example, when a player imparts a spin to a ball, the positioning and movement of the racquet 10 causes a lateral or cross force to be applied to the strings 16 that contact the ball. However, the strings 16 only shift laterally relative to the racquet frame 14 if the sum of the static friction forces at the intersections or junctions 22 between the vertical and horizontal strings, 20 and 18, respectively, is lower than the lateral force imparted by contact with the ball. Thus, to cause the strings to slide, the ball must be hit hard enough to overcome static friction forces. The greatest spin can be imparted to a ball when the strings do not slide at all. However, if the strings are displaced and then return to their pre-displacement position, a greater amount of spin is induced than if the strings are displaced and remain displaced.
With respect to spin control, the ideal string 16 would be under normal tensile loading, yet be easily displaced upon contact with the ball and then quickly rebound to its normal position prior to displacement. This, however, is not how the strings 16 in prior art racquets 10 are configured or behave.
Prior art strings 16, such as those described in U.S. Pat. Nos. 4,005,863 to Henry and 4,377,288 to Sulprizio teach inhibition of string displacement or sliding by providing non-circular strings with angular edges that increase friction at string crossing junctions. Because these strings, as well as more conventional circular strings, do not slide easily once displaced from an initial equidistant string spacing during play, they tend to maintain their irregular spacing until manually relocated by a player during a pause in the action. Until realigned, the irregular weave density causes unpredictable ball rebound.
When the strings 16 are moved by contact with a ball, the string movement transmits less lateral force into the frame 14 of the racquet 10. If the vertical strings 20 do not slide, the entire lateral load is carried by a lateral deflection of the racquet frame 14 followed by frame oscillations or vibrations which are felt in the racquet handle 12. Racquet vibrations thus transmitted to a player's hand and arm can be distracting and fatiguing, and over a prolonged period, excessive vibration can aggravate a condition of "tennis elbow." Unacceptable vibration can be countered by having the racquet 10 strung at a lower tension, because looser strings 16 provide a greater cushion for the ball load. But, as discussed hereinabove, a reduction in string tension exacts a rebound and control penalty.
If a player opts for normal (high) string tension in a conventionally strung racquet 10, rapid string wear results in addition to vibration and control problems. Conventional strings 16 develop notches 24 in the strings 16 as shown in FIG. 3, from wear caused by high string tension in the woven string pattern, ball contact increasing the normal force at the string junctions 22, and lateral movement of the vertical strings 20 caused by lateral forces imparted to the ball. The tensile load carried by the strings 16 is channeled to the reduced cross-sectional string area in the vicinity of the notches 24. These stress concentrations significantly increase string elongation in these areas and shorten the life of the strings 16. Most often, string failure occurs in the vicinity of the notches 24 in the vertical strings 20. This is particularly problematic in newer tennis racquets that have a larger contact surface than in previous racquet configurations. The resultant greater expanse of string 16 necessitates higher string tension which in turn reduces string durability.
In order to ameliorate the string wear problem caused by high tension and exacerbated by ball impact, prior art devices have focused on alteration of string shape to enlarge the contact area or junction between crossed strings in order to reduce wear. For example, U.S. Pat. No. 4,597,576 to Haythornwaite, teaches non-circular cross-section strings that are held more firmly in place by virtue of their shape, to prolong string life by reducing wear due to sliding. Japanese Patent Document No. 60-77776 discloses gut made from synthetic resin that also has a non-circular cross-section. The larger contact area at the string junctions created by the enlarged cross-section reduces string breakage due to tension and increases contact friction to prevent breakage due to hitting impact. German Patent Document No. 3447608 teaches natural gut or plastic strings having an oval or rectangular cross-section, wherein the non-traditional shape increases the contact area between crossed strings so as to reduce high spot friction due to ball impact. While these devices address the problem of string wear, they teach immobilization of the string to maintain the strings in a static relationship and do not consider vibration problems.
Another technique for prolonging string life is the application of a coating, such as a lubricant, to the strings. For example, U.S. Pat. No. 4,377,620 to Alexander, teaches gut that comprises a gut body coated with a film of minute particles of ethylene tetrafluoride resin either in a solvent or a molten resin. The coated gut protects the gut from damage at the time of stretching the gut on the racquet and from wear during use. U.S. Pat. No. 2,307,470 to Slathe, Jr., teaches a gut string coated with a thin nylon layer to improve wear resistance. Neither Alexander nor Slathe, Jr., however, teach coating a non-circular string, nor do they seek to encourage string sliding or teach a structure for reducing racquet vibration.
In summary, none of the prior art discloses or suggests advantages or motivations for combining non-circular strings with a lubricated string to reduce string wear. Furthermore, the prior art does not disclose or suggest how string shape and coating can be configured for improving spin control and reducing vibration without reducing string tension.